1. Field of the Invention
The present invention relates to a digital audio broadcasting receiver. In particular, the present invention relates to a digital audio broadcast receiver for receiving digital audio broadcast signals, e.g., Eureka 147 digital audio broadcasting (DAB).
2. Description of the Related Art
Conventional digital audio broadcasting receivers have been proposed that are compatible with Eureka 147 DAB system based on an OFDM (orthogonal frequency division multiplex) method, as disclosed in General-purpose and application-specific design of a DAB channel decoder, EBU Technical Review Winter 1993, pp.25-35 and Japanese Laid-Open Publication No. 10-126353.
FIG. 9 illustrates a conventional digital audio broadcasting receiver 2000 based on the OFDM method, which includes an RF circuit 100 for converting a radiofrequency signal received from a digital audio broadcast transmitter into an analog base band signal; an analog-digital (AD) converter 101 for converting the analog base band signal into a digital base band signal via sampling; a null symbol detector 102 for detecting a null symbol from the power envelope of the analog base band signal for determining a position at which to start frame processing for the first transfer frame at the time of receiving; an OFDM demodulator 103 for subjecting each symbol to OFDM demodulation by sequentially extracting, with a predetermined symbol cycle, a predetermined number of samples of null symbols, reference symbols, and data symbols from the digital base band signal which is output from the AD converter 101 and sequentially applying FFT (fast Fourier transform) thereto; a digital demodulator 104 for subjecting the output from the OFDM demodulator 103 to a xcfx80/4 shift DQPSK (differential quadriphase phase shift keying) demodulation; an error correction circuit 105 for performing error correction for the output of the digital demodulator 104; and an audio decoder 106 for extracting from the output of the error correction circuit 105 the audio data which has been compressed on the transmitter side and expanding the audio data into a PCM signal so as to generate audio data which contains plurality of audio samples. The audio data which is output from the audio decoder 106 is reproduced by an audio reproducer (not shown) into sounds.
The conventional digital audio broadcasting receiver 2000 further includes a CIR calculator 107 for calculating the power characteristics of a channel impulse response (hereinafter xe2x80x9cCIRxe2x80x9d) of a transfer path based on the result of the FFT performed for the reference symbols; a VCXO controller 108 for detecting a difference in frequency between the clock signal on the transmitter side and the clock signal on the receiver side based on the calculation results by the CIR calculator 107 for controlling the voltage for a voltage controlled crystal oscillator (hereinafter xe2x80x9cVCXOxe2x80x9d) on the receiver side so as to equalize the clock signal on the receiver side to the clock signal on the transmitter side; a digital-analog (DA) converter 109 for converting the control data from the VCXO controller 108 into an analog signal; a VCXO 110 which is capable of oscillating at various frequencies in accordance with a control voltage that is based on the output from the DA converter 109; and an AD clock signal generator 111 for dividing the clock signal for the VCXO 110 so as to generate a sampling clock signal that defines the sampling cycle of the AD converter 101.
As shown in FIG. 10, one transfer frame TF includes a null symbol TFN which has a very low signal level for indicating the start position of a transfer frame; a reference symbol TFR containing known information; and a plurality of data symbols TFD which represent data for transfer. The digital audio broadcasting receiver 2000 is operable, when starting reception, so as to start a FFT process responsive to the OFDM demodulator 103 receiving the null symbol detection signal from the null symbol detector 102 via a switch 120 which may be controlled by a CPU (not shown). The null symbol TFN, reference symbol TFR, and data symbol TFD which have been output from the AD converter 101 are sequentially subjected to FFT processes by the OFDM demodulator 103, preferably from a central portion of a guard interval of the null symbol TFN, at intervals corresponding to symbols (TFN, TFR, TFD). The reference symbol which has been subjected to the FFT process by the OFDM demodulator 103 and converted into a frequency signal is sent to the CIR calculator 107. In the CIR calculator 107, the reference symbol is multiplied by a conjugate complex number of a known reference symbol, and its result is subjected to an IFFT (inversion fast Fourier transform), whereby the channel impulse response (CIR), which represents the transfer path characteristics along the time axis, is calculated. By calculating the CIR power characteristics, the temporal relationships between a plurality of received waves, e.g., a direct wave and reflected waves, can be known.
As shown in FIG. 11, a direct wave 1101 and reflected waves 1102 are detected from the CIR power characteristics. As shown in FIG. 10, each symbol (TFN, TFR, TFD) has a guard interval (GI) at the beginning for attaining tolerance for the reflected waves 1102. A guard interval GI is a copy of the last xc2xc of each symbol (TFN, TFR, TFD) excluding the guard interval GI. Accordingly, the number of samples of each symbol (TFN, TFR, TFD) is {fraction (5/4)} times as many as the number of samples to be subjected to FFT.
If any reflected waves 1102 are present, the reflected waves 1102 will interfere with a subsequent symbol because the reflected waves 1102 will be delayed behind the direct wave 1101. Accordingly, the OFDM demodulator 103 applies FFT to subsequent symbols so as to ensure that they do not contain any delayed components of preceding symbols, thereby reducing inter-symbol interference and enabling substantially error-free reception. By utilizing the fact that each symbol has a length which is {fraction (5/4)} times the number of samples which need to be subjected to FFT, the FFT may be performed for a portion extracted from the center of the guard interval GI such that no reflected waves 1102 due to any preceding symbol are contained in that portion. As a result, at least those delayed waves which are within xc2xd of the guard interval length are prevented from interfering with subsequent symbols.
In order to ensure that the center of gravity of the CIR power characteristics shown in FIG. 10 is located at the center of the guard interval GI, the VCXO controller 108 controls the clock for the VCXO 110 in the following manner: If the center of the guard interval GI is located temporally before a point which corresponds to xc2xd of the guard interval GI, then the FFT will be performed for a portion which is extracted too late; therefore, the clock for the VCXO 110 is made faster so that an adequately xe2x80x9cearlierxe2x80x9d portion is extracted. Conversely, if the center of the guard interval GI is located temporally after a point which corresponds to xc2xd of the guard interval GI, then the FFT will be performed for a portion which is extracted too early; therefore, the clock for the VCXO 110 is made slower so that an adequately xe2x80x9claterxe2x80x9d portion is extracted. If the first impulse coincides with the center of the guard interval GI, then at least those delayed waves which are within xc2xd of the guard interval length are prevented from causing intersymbol interference.
Thus, by controlling so that the impulse position in the CIR power characteristics coincides with the center of the guard interval GI, intersymbol interference due to reflected waves 1102 can be suppressed. Also, the fixed impulse position means the same DAB transfer frame length for both transmission and reception, which in turn means stable reproduction of audio signals due to synchronization of the receiver-side audio reproduction clock signal with the transmitter-side clock signal.
However, the above-described structure requires the VCXO 110 capable of oscillating at various frequencies and the DA converter 109 for outputting a voltage to be supplied to the VCXO 110 in order to achieve synchronization of the receiver-side clock signal with the transmitter-side clock signal, and these elements can lead to an increase in the manufacturing cost of the digital audio broadcasting receiver. On the other hand, omitting the VCXO 110 that is capable of oscillating at various frequencies and employing instead an oscillator capable of oscillating at a fixed frequency for controlling the digital audio broadcasting receiver would allow any mismatch between the clock signals on the transmitter side and on the receiver side to be reflected in a mismatch between the sampling clock signals (for obtaining audio samples) on the transmitter side and on the receiver side, which hinders synchronization of the receiver-side audio reproduction with the transmitter-side clock signal. For example, if the receiver-side clock signal is faster than the transmitter-side clock signal, audio samples to be reproduced will be depleted, resulting in a disruption in the reproduced sound. On the other hand, if the receiver-side clock signal is slower than the transmitter-side clock signal, the audio decoding processing will lag behind so that some samples may not be appropriately reproduced. In either case, problems such as noise generation may occur.
According to the present invention, there is provided a digital audio broadcasting receiver for receiving a plurality of transfer frames including a first transfer frame and a second transfer frame subsequent to the first transfer frame, each of the plurality of transfer frames containing a null symbol representing a start position of each transfer frame, a reference symbol representing known information, and a data symbol representing data to be transferred, wherein each of the null symbol, the reference symbol, and the data symbol includes a guard interval for preventing intersymbol interference due to a reflected wave, wherein the digital audio broadcasting receiver includes: an analog-digital converter for converting the plurality of transfer frames from an analog signal format into a digital signal format based on a clock signal having a fixed frequency and for outputting the first transfer frame; a demodulator for demodulating the first transfer frame from a first frame processing start position for the first transfer frame; an audio decoder for generating audio data containing a plurality of audio samples based on the data symbol contained in the first transfer frame which has been demodulated by the demodulator; a transfer path characteristics calculator for generating a transfer path characteristics signal representing transfer path characteristics based on the reference symbol contained in the first transfer frame which has been demodulated by the demodulator; a frame processing start position control section for controlling a second frame processing start position for the second transfer frame, by outputting to the demodulator a position control signal representing a difference between a predetermined frame processing reference start position and the first frame processing start position for the first transfer frame based on the transfer path characteristics signal, so that the second frame processing start position for the second transfer frame coincides with the predetermined frame processing reference start position; an audio sample discrepancy calculation section for calculating a discrepancy between a plurality of audio samples contained in audio data to be transmitted by a digital audio transmitter and the plurality of audio samples contained in the audio data generated by the audio decoder based on the position control signal; an audio sample adjustment section for adjusting the number of audio samples contained in the audio data generated by the audio decoder, based on the audio sample discrepancy, and for selectively outputting audio reproduction data; and an audio reproducer for reproducing a sound based on the audio reproduction data which is output from the audio sample adjustment section.
In another embodiment of the invention, the demodulator includes an orthogonal frequency division multiplex demodulator for applying fast Fourier transform to the null symbol, the reference symbol, and the data symbol contained in the plurality of transfer frames, and the transfer path characteristics calculator includes a channel impulse response calculator for generating a channel impulse response power characteristics signal, the channel impulse response power characteristics signal being the transfer path characteristics signal.
In still another embodiment of the invention, the predetermined frame processing reference start position is a predetermined position within the guard interval of the null symbol.
In still another embodiment of the invention, the analog-digital converter converts the plurality of transfer frames from an analog signal format into the digital signal format through sampling with a sampling cycle based on the clock signal having the fixed frequency, and the audio sample discrepancy calculation section includes: a total sample number memory section for storing a total number of samples which have been sampled by the analog-digital converter with the sampling cycle for a predetermined period, the total number of samples corresponding to a difference between the predetermined frame processing reference start position and the first frame processing start position; a sample number conversion section for converting at least some of the total number of samples stored in the total sample number memory section into a number of audio samples, whereby the audio sample discrepancy is calculated; and a total sample number correction section for correcting the total number of samples stored in the total sample number memory section by outputting at least some of the total number of samples after having been converted by the sample number conversion section to the total sample number memory section.
In still another embodiment of the invention, the audio sample adjustment section includes at least one sample adjuster corresponding to at least one of monaural reproduction, stereo reproduction, and multi-channel reproduction, each of the at least one sample adjuster including: an input buffer for storing a predetermined number of audio samples among the audio samples contained in the audio data generated by the audio decoder; a cross-fade processing section for reading the predetermined number of audio samples stored in the input buffer, for performing insertion or deletion of the predetermined number of audio samples with cross-fading, and for generating compensated audio data; a sample adjustment controller for determining a number of audio samples to be inserted or deleted in one process by the cross-fade processing section; and an output selector for selectively outputting the predetermined number of audio samples stored in the input buffer when performing neither insertion nor deletion of audio samples, and selectively outputting the compensated audio data generated by the cross-fade processing section when performing insertion or deletion of audio samples.
In still another embodiment of the invention, the cross-fade processing section includes: a first variable gain amplifier and a second variable gain amplifier; a gain controller for controlling a gain of the first variable gain amplifier and a gain of the second variable gain amplifier; an adder for adding outputs of the first variable gain amplifier and the second variable gain amplifier; and an address generator for generating two addresses for audio samples to be input to the first variable gain amplifier and the second variable gain amplifier for insertion or deletion of the number of audio samples determined by the sample adjustment controller, the two addresses being output to the input buffer, and the gain controller controls the gain of the first variable gain amplifier so as to take a large value first and then gradually decrease and controls the gain of the second variable gain amplifier so as to take a small value first and then gradually increase.
In still another embodiment of the invention, when the sample adjustment controller performs insertion or deletion of a plurality of audio samples, the insertion or deletion is performed at a plurality of times, with predetermined time intervals therebetween, so that one audio sample is inserted or deleted each time.
Thus, according to the present invention, a plurality of transfer frames are converted from an analog signal format into a digital signal format in accordance with a clock signal having a fixed frequency, thereby obviating a VCXO (voltage controlled crystal oscillator) which has conventionally been required for analog-digital conversion. As a result, the manufacturing cost for the digital audio broadcasting receiver according to the present invention can be reduced. Nonetheless, each symbol can be demodulated satisfactorily by controlling a position from which to start demodulation for a transfer frame by means of a frame processing start position controlling section so as to minimize the offset in the position from which to start demodulation for a transfer frame. Furthermore, an audio sample discrepancy due to non-synchronization between a transmitter-side clock signal and a receiver-side clock signal can be compensated for by an audio sample discrepancy calculation section and an audio sample adjustment section. As a result, digital audio signal can be reproduced without requiring synchronization between the transmitter-side clock signal and the receiver-side clock signal.
A receiver which is compatible with Eureka 147 DAB system can be realized by implementing a demodulator in the form of an orthogonal frequency division multiplex demodulator for performing a fast Fourier transform, and implementing a transfer path characteristics calculator in the form of a channel impulse response calculator for generating a power characteristics signal for the channel impulse response as a transfer path characteristics signal.
Furthermore, by ensuring that a frame processing start position is located at a predetermined position within a guard interval of a null symbol, it becomes possible to prevent any delayed reflected waves from exerting unwanted influences on the demodulation of a subsequent transfer frame.
Thus, the invention described herein makes possible the advantage of providing a digital audio broadcasting receiver incorporating an oscillator for oscillation at a fixed frequency, which is capable of controlling an OFDM processing position so as to prevent intersymbol interference due to reflected waves, and which is capable of compensating for audio sample mismatching due to any mismatch between the clock signal on the transmitter-side and the clock signal on the receiver-side, so that audio data can be stably reproduced.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.